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    <div class="post-body" itemprop="articleBody"><h2 id="using用法"><a href="#using用法" class="headerlink" title="using用法"></a>using用法</h2><ol>
<li><p>导入命名空间</p>
 <figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 导入整个命名空间到当前作用域，这样std::cout就可以写成cout了。</span></span><br><span class="line"><span class="keyword">using</span> <span class="keyword">namespace</span> std;</span><br><span class="line"><span class="comment">// 只导入某个变量到当前作用域 </span></span><br><span class="line"><span class="keyword">using</span> std::cout; </span><br></pre></td></tr></table></figure>

<p> 一般头文件中要避免使用using导入命名空间。否则引入这个头文件的也相当于导入了命名空间，容易带来命名冲突。</p>
</li>
<li><p>指定别名，C++11中可以使用using来指定别名，作用与typedef相同</p>
 <figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">typedef</span> <span class="type">int</span> T; <span class="comment">// 用 T 代替 int</span></span><br><span class="line"><span class="keyword">using</span> T = <span class="type">int</span>; <span class="comment">// 用 T 代替 int</span></span><br></pre></td></tr></table></figure>
</li>
<li><p>派生类引入基类成员</p>
 <figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">Base</span> &#123;</span><br><span class="line"><span class="keyword">protected</span>:  <span class="comment">//此处可以改成public，但不能改成private</span></span><br><span class="line">    <span class="type">int</span> value;</span><br><span class="line">    <span class="type">int</span> name;</span><br><span class="line">&#125;;</span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">Derived</span> : <span class="keyword">private</span> Base &#123; <span class="comment">//Base是私有继承</span></span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line">    <span class="comment">//通过Derived的实例可以访问父类的value，但是无法访问父类的name变量</span></span><br><span class="line">    <span class="keyword">using</span> Base::value;  </span><br><span class="line">&#125;;</span><br></pre></td></tr></table></figure>

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<h2 id="类型转换"><a href="#类型转换" class="headerlink" title="类型转换"></a>类型转换</h2><h3 id="static-cast"><a href="#static-cast" class="headerlink" title="static_cast"></a><a target="_blank" rel="noopener external nofollow noreferrer" href="https://en.cppreference.com/w/cpp/language/static_cast">static_cast</a></h3><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">B</span> &#123;&#125;;</span><br><span class="line"><span class="keyword">class</span> <span class="title class_">D</span> : <span class="keyword">public</span> B &#123;&#125;;</span><br><span class="line"></span><br><span class="line">D d;</span><br><span class="line">B&amp; b = d;</span><br><span class="line">D&amp; newD = <span class="built_in">static_cast</span>&lt;D&amp;&gt;(b);</span><br></pre></td></tr></table></figure>

<p>代码中D和B的继承关系不是虚继承（没有virtual修饰继承关系），如果是虚继承，就无法使用static_cast了。</p>
<h3 id="dynamic-cast"><a href="#dynamic-cast" class="headerlink" title="dynamic_cast"></a><a target="_blank" rel="noopener external nofollow noreferrer" href="https://en.cppreference.com/w/cpp/language/dynamic_cast">dynamic_cast</a></h3><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">B</span> &#123;</span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line">    <span class="function"><span class="keyword">virtual</span> <span class="type">void</span> <span class="title">test</span><span class="params">()</span> </span>&#123;&#125;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">class</span> <span class="title class_">D</span> : <span class="keyword">virtual</span> <span class="keyword">public</span> B &#123;&#125;;</span><br><span class="line"></span><br><span class="line">D d;</span><br><span class="line">B&amp; b = d;</span><br><span class="line">D&amp; newD = <span class="built_in">dynamic_cast</span>&lt;D&amp;&gt;(b); <span class="comment">// downcast</span></span><br><span class="line"></span><br><span class="line">B&amp; newB = <span class="built_in">dynamic_cast</span>&lt;B&amp;&gt;(d); <span class="comment">//可以成功，但无必要，可以直接使用B&amp; newB = d;效率更高</span></span><br></pre></td></tr></table></figure>

<p>上述代码有两个关键点：1）D与B的继承关系通过virtual修饰；2）B包含虚函数。只有这两个条件同时满足才能使用dynamic_cast。另外与static_cast区别是：static_cast必然成功，而dynamic_cast失败的情况会返回target-type类型的空指针。</p>
<h2 id="值分类"><a href="#值分类" class="headerlink" title="值分类"></a>值分类</h2><p><a target="_blank" rel="noopener external nofollow noreferrer" href="https://en.cppreference.com/w/cpp/language/value_category">Value categories</a>介绍了 lvalue/prvalue/xvalue/glvalue/rvalue</p>
<h2 id="参考"><a href="#参考" class="headerlink" title="参考"></a>参考</h2><p><a target="_blank" rel="noopener external nofollow noreferrer" href="https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines">C++ Core Guidelines</a><br><a target="_blank" rel="noopener external nofollow noreferrer" href="https://learn.microsoft.com/zh-cn/cpp/cpp/cpp-language-reference?view=msvc-170">C++ 语言参考</a></p>

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